Fumed silanized and ground silica

ABSTRACT

The invention relates to a hydrophobic fumed silica which is obtained by grinding a fumed silica which, as a result of silanization, has trimethylsilyl groups fixed on the surface; and to coating formulations comprising this silica.

The invention relates to a hydrophobic fumed ground silica, to a processfor preparing it and to its use. The present invention further relatesto a coating formulation comprising the silica of the invention.

Fumed silica is known from Ullmanns, Enzyklopädie der technischenChemie, volume 21, page 464 (1982). The fumed silica is prepared bycombusting an evaporable silicon compound, for example silicontetrachloride, in a mixture with hydrogen and oxygen.

The comminution of substances to flours (50-500 μm), powders (5-50 μm)and even greater fineness (less than 5 μm) is common practice in theart. For all comminution tasks, a multitude of technical equipment issupplied and operated, which is matched to the peculiarities of theindividual tasks. A good overview of the comminution problems and thevarious machines is given in Ullmanns Enzyklopädie der technischenChemie, 3rd edition, volume 1, page 616 to page 638.

In fumed silica, the mean primary particle diameters are considerablylower (5-50 nm) than can be obtained by a mechanical comminution. Theprimary particles and aggregates of fumed silica with a surface area of200 m²/g can be visualized in an electron microscope.

The primary particles and aggregates of a fumed silica combine to formlarger composites, the agglomerates. In general, the smaller theparticle size is or the greater the specific surface area is, and themore highly compacted the fumed silica is, the larger these agglomeratesare.

The binding forces with which these agglomerates are held together arerelatively weak. Nevertheless, in the incorporation and dissolution ofthese agglomerates in a liquid system for the purpose of homogeneousdistribution of the primary particles and aggregates orlow-agglomeration particles, a certain expenditure of shear energy isrequired. For the dispersion, according to the field of use, a widevariety of different mixing units is used, the crucial factors for theselection being both the viscosity and polarity of the system and theagglomerate strength and the desired homogeneity.

With simple stirrer systems, for example paddle stirrers, directincorporation of small amounts of silicas usually cannot be carried outsatisfactorily, particularly when low-viscosity systems are involved.However, manufacturers of paints and coatings, and also processors, havean interest in achieving an optimal distribution from a performancepoint of view of the silicas which are used predominantly as thickenersand thixotropic agents with very simple equipment and with a minimumlevel of time consumption and energy expenditure.

In the case of paddle stirrer dispersion, the coarse silica agglomeratesare not comminuted sufficiently and can thus make only a smallcontribution to raising viscosities and thixotropy. The information isbased on a UP resin (unsaturated polyester resin) as a dispersant.

A reduction of the agglomerate size by dispersing outside a liquidsystem, i.e., in practical terms, under air or by grinding in theconventional sense, was possible in the prior art only to a limiteddegree, since, in the case of a given agglomeration tendency of thematerial, the old agglomeration state is soon re-established after thecomminution. This effect occurs no later than after recompaction of thematerial which has been greatly loosened by the mechanical interventionand cannot be shipped or stored in this form. The storage time wouldalso have an effect, in the sense of agglomerate reenlargement.

The mass number and evaluation parameter employed for the state ofdistribution of a dispersible silica and maximum agglomerate size of thedispersion (granularity) is the so-called grindometer value to DIN53203.

It is known that fumed silica can be hydrophobized, ground in a pin milland then classified (US 2004/0110077 A1). This known silica is used asan external additive in toner mixtures.

Hydrophilic fumed silica with a BET surface area of 200 m²/g has agrindometer value, determined in UP resin (Ludopal P6 unsaturatedpolyester resin from BASF, 2% dispersion) by the DIN method, of 50 to 60μm.

When this fumed silica is additionally more highly compacted (100 to 120g/l), the grindometer value is also significantly higher, specificallymore than 100 μm, as a result of which an additional, not inconsiderableexpenditure of energy is required as a thickener and thixotropic agent.

It is known that a high-dispersity silica with a surface area of approx.300 m²/g can be ground in a pin mill.

The grindometer value achieved is initially 25 μm for the uncompactedsilica.

If this silica is compacted to 50 g/l, the grindometer value rises to 30μm, and, in the event of further compaction to 75 g/l, even to about 40μm.

In the course of storage over a period of three months, the ground,unmodified silica compacted to 50 g/l has a grindometer value of 50 to60 μm.

According to the prior art, reagglomeration can be prevented only if thehydrophilic silica is mixed with 3% by weight of a hydrophobic silicaand this mixture is ground by means of an air-jet mill or of a pin mill(EP 0 076 377 B1).

For a fumed silica with a BET surface area of 200 m²/g, even aftercompaction to 73 g/l or 107 g/l, a grindometer value of 35 μm isattained.

In the case of a fumed silica with a BET surface area of 300 m²/g, as aresult of the addition of hydrophobic silica before the grinding, thegrindometer value of 10 μm at a tamped density of 28 g/l and of 15 to 20μm at a tamped density of 50 g/l are achieved.

The known fumed silicas have the disadvantage that they still haverelatively high grindometer values and hence their contribution toincreasing the viscosities and thixotropy is not optimal, and that thevalues worsen in the course of prolonged storage.

The technical object was therefore to provide fumed silanized silicaswhich have improved rheological properties and low grindometer values.

The technical object is achieved by a hydrophobic fumed silica which isobtained by grinding a fumed silica which, as a result of silanization,has trimethylsilyl groups fixed on the surface.

Preferably, the grinding is effected with a pin mill or an air-jet mill.This affords silicas which have a lower grindometer value than theunground starting material used, i.e. the unground silicas. The groundsilica of the invention is therefore better and more rapidlydispersible, for example, in coating formulations.

In a preferred embodiment, the silica of the invention has a tampeddensity of 10 to 100 g/l, preferably of 15 to 65 g/l.

Fumed silicas are known from Winnacker-Kuchler Chemische Technologie[Chemical technology], volume 3 (1983), 4th edition, page 77, andUllmanns Enzyklopädie der technischen Chemie, 4th edition (1982), volume21, page 462.

In particular, fumed silicas are prepared by flame hydrolysis ofevaporable silicon compounds, for example SiCl₄, or organic siliconcompounds, such as trichloro-methylsilane.

The silicas in the context of the present invention are fumed silicas,the surface having been modified with at least one organic component.They are therefore referred to as surface-modified silicas. Modifiedfumed silicas (silicas prepared from fumed silicas) are understood tomeans silicas which can be prepared on the basis of fumed silicasaccording to DE 24 14 478. Surface modification is understood to meanthe chemical and/or physical attachment of organic components to thesurface of the silica particles. In other words, in the case ofsurface-modified silicas, at least part of the surface of at least someof the silica particles is covered with the surface modifiers. In thepresent case, the fumed silicas are silanized by reacting fumed silicawith trimethylchlorosilane or trimethylsilanol or hexamethyldisilazanein a known manner, the trimethylsilyl groups being fixed on the surfaceof the fumed silica.

The silica of the invention does not tend to reagglomerate. Thegrindometer value of the silica of the invention is lower than that ofthe starting material and, in the case of a dispersion time of 30 min,is 12 μm or less, while the grindometer value of the unground silica is15 μm. In the case of a dispersion time of 60 min, the grindometer valueis below 10 μm.

In a further preferred embodiment, the silica of the invention has aspecific BET surface area of 150 to 300 m²/g, preferably of 200 to 295m²/g, more preferably of 195 to 290 m²/g.

It is also preferred that the silica of the invention has a meanparticle size according to Cilas of 3.5 to 8.5. In a further preferredembodiment, the silica according to the present invention has a pH inthe range of 5.0 to 8.0.

The silica of the invention preferably has an agglomerate strength ofless than 25 mm, more preferably of less than 20 mm.

The invention further provides a process for producing the silica of theinvention, the process being characterized by the step of grinding afumed silica which, as a result of silanization, has trimethylsilylgroups fixed on the surface.

In a preferred process, the silica used has a BET surface area of 150 to350 m²/g, preferably of 180 to 300 m²/g and a tamped density of 50 to100 g/l, preferably of 55 to 65 g/l and more preferably of approx. 60g/l.

In a further preferred embodiment of the preparation process accordingto the present invention, the silicas used have the followingphysicochemical characteristic data:

BET surface area m²/g: 150-300 Mean size of the primary 7 particles nm:pH: 5.0-9.0 carbon content % by weight: 0.1 to 10, preferably 2.0-4.0

In a further preferred preparation process, the silicas used also havethe following physicochemical characteristic data:

Tamped density²⁾ g/l Approx. 60 Drying loss³⁾ (2 h at 105° C.) <=0.5 %by weight on departure from the manufacturer Ignition loss⁴⁾⁵⁾ (2 h at1000° C.) 1.0-3.0 % by weight pH⁶⁾⁷⁾ 5.5-7.5 SiO₂ content⁸⁾ % byweight >=99.8 Al₂O₃ content⁸⁾ % by weight <=0.05 Fe₂O₃ content⁸⁾ % byweight <=0.01 TiO₂ content⁸⁾ % by weight <=0.03 HCl content⁸⁾⁹⁾ % byweight <=0.025 ¹⁾To DIN ISO 9277 ²⁾To DIN EN ISO 787-11, JIS K 5101/20(unscreened) ³⁾To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23 ⁴⁾To DINEN 3262-20, ASTM D 1208, JIS K 5101/24 ⁵⁾Based on the substance dried at105° C. for 2 hours ⁶⁾To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26⁷⁾Water:methanol = 1:1 ⁸⁾Based on the substance calcined at 1000 C. for2 hours ⁹⁾HCl content is part of the ignition loss

The silicas of the invention are used as thickeners or thixotropicagents in coating formulations.

The present invention therefore also provides coating formulationscomprising a hydrophobic fumed silica which is obtained by grinding afumed silica which, as a result of silanization, has trimethylsilylgroups fixed on the surface. The ground silica present has a lowergrindometer value than the unground silica. The grindometer value may beless than 12 μm.

In a preferred embodiment, the tamped density of the silica is 10 to 100g/l, preferably 15 to 65 g/l.

Coating formulations in the context of the present invention are coatingformulations comprising at least one polymer component and/or a mixtureof a plurality of physically or chemically crosslinking polymercomponents, at least one solvent and at least one surface-modifiedsilica. The coating formulations of the invention are preferably1-component coatings, 2-component coatings and UV coatings, especiallypolyurethane coatings, and most preferably clearcoats and matt coatingformulations.

A clearcoat in the sense of the invention is a coating material which,applied on a substrate, forms a transparent coating with protective,decorative or specifically technical properties. In a coating system,the clearcoat protects, as the uppermost layer, the layers below it frommechanical damage and weathering influences. A clearcoat does notcomprise any pigments. Especially in the case of clearcoats, thetransparency of the coating, i.e. the visual impression of how clear andundistorted the surface of the material coated with the clearcoat can beseen through the coating after it has dried, is a measure of the qualityof the coating. When the clearcoat is applied on a shiny blackbackground, the blackness value M_(y) can be employed as a measure forthe transparency of this coating.

In a preferred embodiment, the coating formulation at a reflectometervalue of 85 to 90, preferably of 87 to 88, has a blackness value M_(y)of at least 280, preferably of at least 285.

It is also preferred that the coating formulation comprises 0.5 to 15%by weight of the silica.

In addition to the components mentioned, the coating formulations of theinvention may also comprise further assistance and additives typicallyused in coatings, for example plasticizers, stabilizers, phasemediators, pigments, surfactants, desiccants, catalysts, initiators,photosensitizers, inhibitors, light stabilizers and preservatives.

Coating formulations of the invention may, as binders, comprise theresins customary in paints and coatings technology, as described, forexample, in “Lackharze, Chemie, Eigenschaften and Anwendungen [Coatingresins, chemistry, properties and applications], Eds. D. Stoye, W.Freitag, Hanser Verlag, Munich, Vienna 1996”. The contents of thispublication are hereby incorporated explicitly into the content of thedescription of the present invention. Examples include the polymers andcopolymers of (meth)acrylic acid and their esters, which optionally bearfurther functional groups, with further olefinically unsaturatedcompounds, for example styrene; polyetherpolyols, polyesterpolyols,polycarbonatepolyols, polyurethanepolyols and epoxy resins, and anydesired mixtures of these polymers, and also fatty acid-modified “alkydresins” prepared by polycondensation, as described in Ullmann, 3rdedition, volume 11, page 334 ff. The contents of this publication arehereby incorporated explicitly into the content of the description ofthe present invention.

Preference is given to using, as polymer components, organic compoundsbearing hydroxyl groups, for example polyacrylatepolyols,polyesterpolyols, polycaprolactonepolyols, polyetherpolyols,polycarbonatepolyols, polyurethanepolyols and hydroxy-functional epoxyresins, and any mixtures of these polymers. The particularly preferredpolymeric organic compounds used are aqueous or solvent-containing orsolvent-free polyacrylatepolyols and polyesterpolyols and any mixturesthereof.

Suitable polyacrylatepolyols are copolymers of, inter alia, monomershaving hydroxyl groups with other olefinically unsaturated monomers, forexample esters of (meth)acrylic acid, styrene, [alpha]-methylstyrene,vinyltoluene, vinyl esters, maleic and fumaric mono- and dialkyl esters,[alpha]-olefins and further unsaturated oligomers and polymers.

In a further preferred embodiment, the coating formulation comprises 5.0to 99.5% by weight of solids of a polymer component or of a mixture oftwo or more physically or chemically crosslinking polymer componentsand/or 0 to 99.5% by weight of a low molecular weight component whichfunctions as a solvent or of a mixture of such low molecular weightcomponents.

It is also preferred that the coating formulation comprises at least onebinder selected from the group consisting of polymers and copolymers of(meth)acrylic acid and esters thereof, which optionally bear furtherfunctional groups, with further olefinically unsaturated compounds, forexample styrene; polyetherpolyols, polyesterpolyols,polycarbonatepolyols, polyurethanepolyols, epoxy resins, and fattyacid-modified alkyd resins prepared by polycondensation.

The present invention is illustrated with reference to the exampleswhich follow, which do not, however, restrict the scope of protection.

EXAMPLES

1. Grinding

To prepare the inventive examples, commercial AEROSIL® R812 or AEROSIL®R8125 (sack goods) were metered into the mill used with a meteringbalance and ground. The physicochemical characteristic data of theAEROSIL® R812 or AEROSIL® R8125 are listed in Table 1.

TABLE 1 Fumed silica used AEROSIL ® R812 AEROSIL ® R8125 Behaviourtowards water Hydrophobic Hydrophobic Appearance White powder Whitepowder BET surface area¹⁾ 230-290 195-245 m²/g Mean size of the primary7 7 particles nm Tamped densities²⁾ g/l Approx. 60 Approx. 60 Dryingloss³⁾ (2 h at 105° C.) <=0.5 <=0.5 % by weight on departure from themanufacture Ignition loss⁴⁾⁵⁾ (2 h at 1.0-2.5 1.5-3.0 1000° C.) % byweight Carbon content % by weight 2.0-3.0 3.0-4.0 pH⁶⁾⁷⁾ 5.5-7.5 5.5-7.5SiO₂ content⁸⁾ % by weight >=99.8 >=99.8 Al₂O₃ content⁸⁾ & by weight<=0.05 <=0.05 Fe₂O₃ content⁸⁾ & by weight <=0.01 <=0.01 TiO₂ content⁸⁾ &by weight <=0.03 <=0.03 HCl content⁸⁾⁹⁾ & by weight <=0.025 <=0.025 ¹⁾ToDIN ISO 9277 ²⁾To DIN EN ISO 787-11, JIS K 5101/20 (unscreened) ³⁾To DINEN ISO 787-2, ASTM D 280, JIS K 5101/23 ⁴⁾To DIN EN 3262-20, ASTM D1208, JIS K 5101/24 ⁵⁾Based on the substance dried at 105° C. for 2hours ⁶⁾To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26 ⁷⁾Water:methanol= 1:1 ⁸⁾Based on the substance calcined at 1000 C. for 2 hours ⁹⁾HClcontent is part of the ignition loss

For the tests, a pin mill (Alpine 160Z, rotor diameter 160 mm) of anair-jet mill (grinding space diameter: 240 mm, grinding space height: 35mm) was used. The ground product was isolated with a bag filter (filterarea: 3.6 m², filter material: nylon fabric). In further tests, theresulting ground product was packed into commercial sacks with acommercial bagging machine. In further tests, the sacks packed withground product were levelled with a technically customary methodsuitable for this purpose before palleting. The levelled sacks were, asis commercially customary, palleted and subsequently stored over fiveweeks. The parameters of the production process are listed in Table 2.

TABLE 2 The table shows the parameters of the production of someexamples of the silica of the inventions GA** GA** IA*** IA*** ratepressure rate pressure Metering Example Mill* [m³] [bar] [m³] [bar][kg/h] Bagging Levelling Storage 1 AJ 27.5 3.5 15.9 3.7 10 No No No 2 AJ27.5 3.5 15.9 3.7 10 Yes No No 3 AJ 27.5 3.5 15.9 3.7 10 Yes Yes No 4 AJ27.5 3.5 15.9 3.7 10 Yes Yes Yes 5 AJ 11.5 1.0 6.8 1.2 10 No No No 6 AJ11.5 1.0 6.8 1.2 10 Yes No No 7 AJ 11.5 1.0 6.8 1.2 10 Yes Yes No 8 AJ11.5 1.0 6.8 1.2 10 Yes Yes Yes 9 PM — — — — 10 No No No 10 PM — — — —10 Yes No No 11 PM — — — — 10 Yes Yes No 12 PM — — — — 10 Yes Yes Yes 13PM — — — — 20 No No No 14 PM — — — — 20 Yes No No 15 PM — — — — 20 YesYes No 16 PM — — — — 20 Yes Yes Yes *AJ = air-jet mill; PM = pin mill,GA** = grinding air; IA*** = injector air

2. Determination of the Physicochemical Characteristic Data of theGround Silicas

2.1 BET Surface Area

The BET surface area is determined to DIN ISO 9277.

2.2 Tamped Density

The tamped density was determined to DIN EN ISO 787-11. A defined amountof a sample which had not been screened beforehand is filled into agraduated glass cylinder and subjected to a fixed number of tampingoperations by means of a tamping volumeter. During the tamping, thesample is compacted. As a result of the analysis carried out, the tampeddensity is obtained.

Fundamentals of the Tamped Density Determination:

The tamped density (formerly tamped volume) is equal to the quotient ofthe mass and the volume of a powder after tamping in a tamping volumeterunder fixed conditions. According to DIN ISO 787/XI, the tamped densityis reported in g/cm³. Owing to the very low tamped density of theoxides, however, the value is reported here in g/l. In addition, thedrying and screening and the repetition of the tamping operation aredispensed with.

Equipment for Tamped Density Determination:

tamping volumeter

measuring cylinder

laboratory balance (readability 0.01 g)

Performance of the Tamped Density Determination:

200±10 ml of oxide are filled into the measuring cylinder of the tampingvolumeter such that no cavities remain and the surface is horizontal.The mass of the sample introduced is determined accurately to 0.01 g.The measuring cylinder with the sample is placed into the measuringcylinder holder of the tamping volumeter and tamped 1250 times. Thevolume of the tamped oxide is read off accurately to 1 ml.

The Valuation of the Tamped Density Determination:

${{tamped}\mspace{14mu} {density}\mspace{14mu} \left( {g\text{/}l} \right)} = \frac{{Initial}\mspace{14mu} {weight}\mspace{14mu} {in}\mspace{14mu} g \times 1000}{{Volume}\mspace{14mu} {read}\mspace{14mu} {off}\mspace{14mu} {in}\mspace{14mu} {ml}}$

2.3 pH

Reagents for pH Determination:

distilled or demineralized water, pH>5.5

methanol, p.a.

buffer solutions pH 7.00, pH 4.66

Equipment for pH Determination:

laboratory balance (readability 0.1 g)

beaker, 250 ml

magnetic stirrer

stirrer bar, length 4 cm

combined pH electrode

pH measuring instrument

Dispensette, 100 ml

Procedure for Determining the pH:

the determination was effected to DIN EN ISO 787-9.

Calibration: before the pH measurement, the measuring instrument iscalibrated with the buffer solutions. When a plurality of measurementsare carried out in succession, a single calibration is sufficient.

4 g of oxide are converted to a paste with 48 g (61 ml) of methanol in a250 ml beaker, and the suspension is diluted with 48 g (48 ml) of waterand stirred with a magnetic stirrer in the presence of an immersed pHelectrode (speed approx. 1000 min⁻¹).

After the stirrer has been switched off, the pH is read off after a waittime of one minute. The result is displayed with one decimal place.

Table 3 summarizes the physicochemical data of the silica of theinvention and of the comparative example.

TABLE 3 Physicochemical data of the silicas of the invention SpecificBET Tamped Median Agglomerate surface area density [nm] strength*Designation [m²/g] pH [g/l] (Cilas) [mm] Comparative 219 7.0 59 16.47 25example Example 1 216 7.4 22 3.7 n.m. Example 2 216 7.5 65 3.69 16Example 3 218 7.7 28 3.68 16 Example 4 218 6.7 51 3.74 16 Example 5 2177.5 20 8.05 n.m. Example 6 217 7.5 59 8.11 13 Example 7 217 7.5 54 8.2216 Example 8 217 7.1 62 7.35 17 Example 9 218 7.5 20 5.85 n.m. Example10 217 7.5 56 6.32 12 Example 11 219 7.2 56 6.03 12 Example 12 218 7.148 5.93 16 Example 13 217 6.9 17 7.18 n.m. Example 14 218 7.4 40 7.0 12Example 15 218 7.2 65 7.23 17 Example 16 218 7.0 55 6.66 16 *n.m. = notmeasurable

3. Performance Tests

3.1 Test Procedure

The patterns described in the examples were performance-tested in a 2KPU clearcoat based on acrylate/isocyanate in comparison to ungroundstarting material. The raw materials used were as follows: Macrynal SM510n CH: 130010625 (Surface Specialities), Desmodur N 75 MPA (Bayer)

Table 4 shows the formulation of the 2K PU clearcoat comprising theground silicas of the invention

TABLE 4 Formulation of the 2K PU clearcoat Parts by wt. MillbaseMacrynal SM 510n 60 LG 23.34 Butyl acetate 98% 8.48 AEROSIL ® 0.70Letdown Millbase 32.52 Macrynal SM 510n 60 LG 33.34 Xyrene 3.92Ethoxypropyl acetate 3.46 Butylglycol acetate 1.50 Butyl acetate 98%3.93 Curing agent Desmodur N 75, 75% strength 21.33 Σ 100.00

Table 5 shows the steps for producing and testing the 2K PU clearcoat.

TABLE 5 Manufacture and testing of the 2K PU clearcoat Predispersiondisperse 2.5 times the amount of millbase with a dissolver at 2500 rpmfor 5 min Dispersion 45 min in a Skandex disperser 250 ml glass bottleswith addition of 200 g of glass beads, grindometer determination after30 and 60 min Letdown with an initial charge of the millbase, theletdown mixture (Macrynal with the remaining components) is added. Thehomogenization is effected with a paddle stirrer. Addition of the thecuring agent Desmodur N 75 is curing agent added with stirring (1000rpm). Subsequently, the mixture is homogenized for 1 min. Applicationspray application at 21 s DIN 4 mm on to metal sheets painted black(DT36) with a spraying machine Setting: 1 crosscoat at setting 3.8; drylayer thickness: approx. 40 μm Spray dilution: Xylene 50 Ethoxypropylacetate 6 Butylglycol acetate 6 Butyl acetate 98% 38 Drying approx. 24 hat RT, then at 70° C. for conditions 2 h 20° the shine and thecloudiness are reflectometer assessed on coating films which have value,haze been applied to black metal sheets with a reflectometer from BykGardner Blackness value the blackness value is determined on M_(y)coating films which have been (assessment of applied to metal sheetspainted transparency) black, with a D19C densitometer from GretagMacbeth. The blackness value M_(y) is obtained by multiplying the valuemeasured by one hundred Wave scan the profile is assessed by means of(profile) a wave-scan plus system from Byk-Gardner

3.2 Grindometer Value

3.2.1 Basics

The degree of dispersion determines the performance properties of theAerosil-thickened liquid. The measurement of the grindometer valueserves for assessment of the degree of dispersion. The grindometer valueis understood to mean the interface layer thickness below which thespots or aggregates present become visible on the surface of the exposedsample.

The sample is exposed with a scraper in a groove whose depth at one endis twice as great as the diameter of the largest Aerosil grains anddecreases constantly to at the other end. On a scale which specifies thedepth of the groove, the value of the depth in micrometres below which arelatively large number of Aerosil grains become visible as a result ofspots or scratches on the surface of the binder system is read off. Thevalue read off is the grindometer value of the present system.

3.2.2. Performance of the Grindometer Value Determination

The grindometer block is placed onto a flat, slip-resistant surface andwiped clean immediately before the test. The Aerosil dispersion, whichmust be free of air bubbles, is then applied at the lowest point in thegroove such that it flows away a little above the edge of the groove.The scraper is then gripped with both hands and placed onto the end ofthe groove in which the dispersion is present perpendicularly to thegrindometer block and at right angles to its longitudinal edges withgentle pressure. The dispersion is then exposed by slow, homogenouspulling of the scraper over the block in the groove. No later than 3seconds after the exposure of the dispersion, the grindometer value isread off.

In the determination, the surface of the exposed dispersion (at rightangles to the channel) is viewed obliquely from above at an angle of20-30° (to the surface). The block is held to the light such that thesurface structure of the exposed dispersion is readily discernible.

On the scale, the grindometer value read off is the value in micrometresbelow which a relatively large number of Aerosil grains become visibleas spots or scratches on the surface. Individual spots or scratcheswhich appear coincidentally are not taken into account.

The granularity is assessed at least twice, and in each case on a newlyexposed dispersion.

3.2.3 Evaluation

The measurements are used to form the arithmetic means. There exists thefollowing relationship between the grindometer value in micrometres andthe Hegmann units and FSTP units based on the target system:

B=8−0.079 A

C=10−0.098 A=1.25 B

In the equations:

A=grindometer value in micrometres

B=grindometer value in Hegmann units

C=grindometer value in FSTP units

Table 6 shows the dispersibility of the silicas of the invention withreference to the grindometer values measured.

TABLE 6 Dispersibility Grindometer value (μm) of the millbases 30 minStarting material/ 15 Comparative example Example 1 12 Example 2 10Example 3 10 Example 4 10 Example 5 12 Example 6 12 Example 7 12 Example8 12 Example 9 12 Example 10 12 Example 11 12 Example 12 12 Example 1312 Example 14 12 Example 15 12 Example 16 12

3.3 Optical Properties

3.3.1 Determination of the 20° Reflectometer Value, Haze

In order to assess any influence on the shine and on the haze throughthe presence of the silica, the 20° reflectometer value is measured. Thereflectometer value is thus an important criterion for thecharacterization of coating films.

3.3.2 Determination of the Transparency as the Blackness Value M_(y)

The blackness value M_(y) is determined on coating films which have beenapplied to metal sheets painted black, using a D19C densitometer fromGretag Macbeth. The value M_(y) gives a statement about the colour depthand transparency of the clearcoat. The higher this value is, the moretransparent is the coating. Simultaneously, the colour depth increases.

Table 7 summarizes the results of the optical properties of the 2K PUclearcoats comprising the silicas of the invention.

TABLE 7 Optical properties 20° reflectometer Blackness Wave scan valueHaze value My L S Starting 87.8 11 286 23 28 material Example 4 87.8 10288 22 28 Example 8 87.8 10 285 24 29 Example 12 87.7 11 286 29 28Example 16 87.8 10 287 26 33

The data of the ground products show lower grindometer values withvirtually equal specific surface areas. Surprisingly, the lowergrindometer values are maintained in spite of compaction, recognizableby the tamped density, as a result of bagging or bagging/levelling andbagging/levelling/storage. In some cases, the tamped densities are evenabove that of the oxide used, i.e. the oxides of the invention possess,in spite of equal or even higher compaction, lower grindometer values.The measurements which were carried out on the coating films comprisingthe silicas of the invention show that the quality criteria of shine,haze and transparency are satisfied with simultaneously improvedproperties with regard to the rheology and dispersibility of the silicasin the coating formulations.

1. A hydrophobic fumed silica which is obtained by grinding a fumedsilica which, as a result of silanization, has trimethylsilyl groupsfixed on the surface.
 2. The silica according to claim 1, wherein thegrinding is effected with a pin mill or an air-jet mill.
 3. The silicaaccording to claim 1, wherein the silica has a lower grindometer valuethan the unground silica.
 4. The silica according to claim 1, whereinthe silica has a tamped density of 10 to 100 g/l.
 5. The silicaaccording to claim 1, wherein the silica has a specific BET surface areaof 200 to 250 m²/g.
 6. The silica according to claim 1, wherein thesilica has a mean particle size according to Cilas of 3.5 to 8.5.
 7. Thesilica according to claim 1, wherein the silica has a pH in the range of6.5 to 8.0.
 8. The silica according to claim 1, wherein the silicaobtained has an agglomerate strength of less than 25 mm.
 9. A processfor producing the fumed silanized silica according to claim 1,comprising grinding a fumed silica which, as a result of silanization,has trimethylsilyl groups fixed on the surface.
 10. The processaccording to claim 9, wherein the silica used for the grinding has a BETsurface area of 150 to 350 m²/g and a tamped density of 50 to 100 g/l.11. (canceled)
 12. A coating formulation comprising a hydrophobic fumedsilica which is obtained by grinding a fumed silica which, as a resultof silanization, has trimethylsilyl groups fixed on the surface.
 13. Thecoating formulation according to claim 12, wherein the ground silicapresent has a lower grindometer value than the unground silica.
 14. Thecoating formulation according to claim 12, wherein the tamped density ofthe silica is 10 to 100 g/l.
 15. The coating formulation according toclaim 12, further comprising a silica which is obtained by grinding afumed silica which, as a result of silanization, has trimethylsilylgroups fixed on the surface.
 16. The coating formulation according toclaim 12, comprising a 20° reflectometer value of 85 to 90, and ablackness value M_(y) of at least
 280. 17. The coating formulationaccording to claim 12, comprising 0.5 to 15% by weight of the silica.18. The coating formulation according to claim 12, comprising 5.0 to99.5% by weight of solids of a polymer component or of a mixture of twoor more physically or chemically crosslinking polymer components and/or0 to 99.5% by weight of a low molecular weight component which functionsas a solvent or of a mixture of said low molecular weight components.19. The coating formulation according to claim 12, comprising at leastone binder selected from the group consisting of polymers and copolymersof (meth)acrylic acid and esters thereof, which optionally bear furtherfunctional groups, with further olefinically unsaturated compounds, forexample styrene; polyetherpolyols, polyesterpolyols,polycarbonatepolyols, polyurethanepolyols, epoxy resins, and fattyacid-modified alkyd resins prepared by polycondensation.
 20. The silicaaccording to claim 5, wherein the silica has a specific BET surface areaof 210 to 225 m²/g.